Okla. Stat. tit. 12, § 12-1458: Failure to answer - New matter in answer not conclusive

If no answer be made, a peremptory mandamus must be allowed

against the defendant; if answer be made, containing new matter, the

same shall not, in any respect, conclude the plaintiff, who may, on

the trial or other proceeding, avail himself of any valid objections

to its sufficiency, or may countervail it by proof, either in direct

denial or by way of avoidance.